Add doc directory and docca submodule

doc/qbk/quickref.xml

@@ -0,0 +1,14 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!DOCTYPE library PUBLIC "-//Boost//DTD BoostBook XML V1.0//EN" "../../../../tools/boostbook/dtd/boostbook.dtd">
+
+<!--
+  Copyright (c) 2016-2019 Vinnie Falco (vinnie dot falco at gmail dot com)
+
+  Distributed under the Boost Software License, Version 1.0. (See accompanying
+  file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+  Official repository: https://github.com/boostorg/beast
+-->
+
+<informaltable frame="all">
+</informaltable>